# 3.5.1.3.20 Bessel_y1

## Definition:

$y1 = bessel\_y1(x)$ evaluates the Bessel function of the second kind,$Y_1$ , x > 0.

The approximation is based on Chebyshev expansions.

For more information please review the s17adc function in the NAG document.

## Parameters:

x (input, double)
The argument x of the function.
Constraint: x>0.0
$y1$ (output, double)
The approximation of the Bessel function of the second kind.